The present invention relates to imaging devices. More specifically, the present invention relates to an exposure control for an optical scanner.
A typical optical scanner uses a motor drive to move a charge coupled device (xe2x80x9cCCDxe2x80x9d) across a sheet of paper. In a color copier or flatbed scanner, for instance, the CCD is moved along the sheet of paper at a constant velocity. As the CCD is being moved, lines of pixels are generated. Each line of pixels is given a constant exposure time; therefore, the exact size of the pixels that make up a scanned image is a direct function of the motion accuracy of the drive system.
The motor drive may include a step motor, which can be controlled to a high positional accuracy. However, an inexpensive step motor can cause residual position errors to appear in scanned images as xe2x80x9ccolor fringesxe2x80x9d around black-to-white edges. Color fringes and xe2x80x9cjaggiesxe2x80x9d can degrade image quality and they can severely reduce the accuracy of text recognition software, such as optical character recognition (xe2x80x9cOCRxe2x80x9d) programs.
There are other limitations associated with step motors. A step motor can be noisy, it has a limited range of speeds over which it can perform acceptably, and its power consumption is relatively high. Additionally, motion of the step motor is inherently oscillatory because it is driven by discrete step inputs. The oscillatory motion can cause color fringes in a scanner using a three-channel RGB CCD.
Scan motion accuracy is often limited by the finite step size. Even if the step motor is microstepped to improve resolution and reduce torque ripple, some motion errors are still inevitable.
An inexpensive step motor drive system does not have position or velocity feedback, so there is no way of knowing the true position of the CCD or whether the step motor has stalled. Therefore, the step motor is typically overdriven with a considerable amount of extra torque to satisfy the worst case load. Overdriving increases the size, cost and heat dissipation of the motor driver.
Assignee""s U.S. Pat. No. 6,037,584 discloses scanners that include dc motor drives instead of step motor drives. These scanners are accurate enough to produce quality images and meet the cost, size, weight, and power requirements of low cost commercial scanning products such as flatbed scanners, scrollfed scanners, color copiers, fax machines and all-in-one multi-function products. The dc motors have a smaller size, quieter operation and smoother motion than step motors used in similar products. The dc motors also have a broader speed range (including higher speeds), lower power consumption, and simpler drive electronics. Additionally, the dc motors are lower in cost at an equivalent level of performance.
According to one aspect of the present invention, an optical scanning system includes a photodetector including at least one channel; and a non-linear gain compensation control for at least one channel of the photodetector. Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the present invention.